The broad objective of this research is continuing observation in detail of the demographic transition in a developing country, Taiwan, with special attention to trends and differentials in fertility. Another focus is the role of social intervention to speed the fertility decline through an organized family planning program. Detailed knowledge of the interaction of social and economic changes, with changes in the biosocial system of reproduction, cannot be fully recaptured from the historical studies of the West. In addition, our long-term analyses of this process in Taiwan have implications for many developing countries and, as a result, are closely followed around the world. Taiwan appears to be entering a new phase of its demographic history. While from 1965 to 1973 there was a sharp upward trend in contraceptive use with dimunition of differentials among social strata, only toward the end of this period was there evidence of a decline in desired number of children. Continuing fertility decline depends on what happens at ages under 30. There are a number of factors contributing to lower fertility, but strong persistence of traditional familial organization and preference for sons may sustain current levels. Detailed assessment of the factors contributing to the rapid increase in contraceptive practice and the degree to which potential for lowered fertility is realized, and among which strata, represent prime objectives of our proposed research. These analyses will be possible through the extensive data files already processed on the demographic characteristics and many of the determinants of fertility, through our ability to follow fertility of individual couples prospectively in the household register at little cost, and through a new methodology for measuring the preference structure for number and sex of children.